DDR3 SDRAM is an improvement over its predecessor, DDR2 SDRAM, and the two are not compatible. The primary benefit of DDR3 is the ability to transfer at twice the data rate of DDR2 (I/O at 8× the data rate of the memory cells it contains), thus enabling higher bus rates and higher peak rates than earlier memory technologies. In addition, the DDR3 standard allows for chip capacities of 512 megabits to 8 gigabits, effectively enabling a maximum memory module size of 16 gigabytes.

Contents

Overview

DDR, DDR2 and DDR3 for Desktop PC's Comparison Graphic

DDR3 memory provides a reduction in power consumption of 16% compared to DDR2 modules due to DDR3's 1.5 V supply voltage, compared to DDR2's 1.8 V or DDR's 2.5 V. The 1.5 V supply voltage works well with the 90 nanometer fabrication technology used in the original DDR3 chips. Some manufacturers further propose using "dual-gate" transistors to reduce leakage of current.[1]

According to JEDEC[2] the maximum recommended voltage is 1.575 volts and should be considered the absolute maximum when memory stability is the foremost consideration, such as in servers or other mission critical devices. In addition, JEDEC states that memory modules must withstand up to 1.975 volts before incurring permanent damage, although they are not required to function correctly at that level.

The main benefit of DDR3 comes from the higher bandwidth made possible by DDR3's 8 burst deep prefetch buffer, in contrast to DDR2's 4 burst deep or DDR's 2 burst deep prefetch buffer.

DDR3 modules can transfer data at a rate of 800–1600 MT/s using both rising and falling edges of a 400–800 MHz I/O clock. In comparison, DDR2's current range of data transfer rates is 400–1066 MT/s using a 200–533 MHz I/O clock, and DDR's range is 200–400 MT/s based on a 100–200 MHz I/O clock. High-performance graphics was an initial driver of such bandwidth requirements, where high bandwidth data transfer between framebuffers is required.

DDR3 prototypes were announced in early 2005. Products in the form of motherboards appeared on the market in June 2007[3] based on Intel's P35 "Bearlake" chipset with DIMMs at bandwidths up to DDR3-1600 (PC3-12800).[4] The Intel Core i7, released in November 2008, connects directly to memory rather than via a chipset. The Core i7 supports only DDR3. AMD's first socket AM3Phenom II X4 processors, released in February 2009, were their first to support DDR3.

DDR3 DIMMs have 240 pins, are electrically incompatible with DDR2 and have a different key notch location.[5] DDR3 SO-DIMMs have 204 pins.[6]

GDDR3 memory, having a similar name but being from an entirely dissimilar technology, has been in use for graphic cards by companies such as NVIDIA and ATI Technologies. GDDR3 has sometimes been incorrectly referred to as "DDR3".

Latencies

While the typical latencies for a JEDEC DDR2 device were 5-5-5-15, the standard latencies for the JEDEC DDR3 devices are 7-7-7-20 for DDR3-1066 and 7-7-7-24 for DDR3-1333.

DDR3 latencies are numerically higher because the I/O bus clock cycles by which they are measured are shorter; the actual time interval is similar to DDR2 latencies (around 10 ns). There is some improvement because DDR3 generally uses more recent manufacturing processes, but this is not directly caused by the change to DDR3.

As with earlier memory generations, faster DDR3 memory became available after the release of the initial versions. DDR3-2000 memory with 9-9-9-28 latency (9 ns) was available in time to coincide with the Intel Core i7 release.[7] CAS latency of 9 at 1000 MHz (DDR3-2000) is 9 ns, while CAS latency of 7 at 667 MHz (DDR3-1333) is 10.5 ns.

Note: All above listed are specified by JEDEC as JESD79-3.[9] All RAM data rates in-between or above these listed specifications are not standardized by JEDEC—often they are simply manufacturer optimizations using higher-tolerance or overvolted chips. Of these non-standard specifications, the highest purported speed reached was equivalent to DDR3-2500.[10]

DDR3-xxx denotes data transfer rate, and describes raw DDR chips, whereas PC3-xxxx denotes theoretical bandwidth (though it is often rounded up or down), and is used to describe assembled DIMMs. Bandwidth is calculated by taking transfers per second and multiplying by eight. This is because DDR3 memory modules transfer data on a bus that is 64 data bits wide, and since a byte comprises 8 bits, this equates to 8 bytes of data per transfer.

In addition to bandwidth and capacity variants, modules can

Optionally implement ECC, which is an extra data byte lane used for correcting minor errors and detecting major errors for better reliability. Modules with ECC are identified by an additional ECC or E in their designation. For example: "PC3-6400 ECC", or PC3-8500E. [11]

Be "registered", which improves signal integrity (and hence potentially clock rates and physical slot capacity) by electrically buffering the signals with a register, at a cost of an extra clock of increased latency. Those modules are identified by an additional R in their designation, whereas non-registered (a.k.a. "unbuffered") RAM may be identified by an additional U in the designation. PC3-6400R is a registered PC3-6400 module, PC3-6400R ECC is the same module but with additional ECC.

Be fully buffered modules, which are designated by F or FB and do not have the same notch position as other classes. Fully buffered modules cannot be used with motherboards that are made for registered modules, and the different notch position physically prevents their insertion.

Are notbackwards compatible—DDR3 modules do not fit into DDR2 sockets; forcing them can damage the DIMM and/or the motherboard[12]

Advantages compared with DDR2

Higher bandwidth performance, up to 1600 MT/s standardized

Slightly improved latencies as measured in nanoseconds

Higher performance at low power (longer battery life in laptops)

Enhanced low-power features

Disadvantages compared with DDR2

As of January 2010, DDR3 memory again costs more than (slower) DDR2 memory[13]

Market penetration

Although DDR3 was launched in 2007, DDR3 sales are not expected to overtake DDR2 until the end of 2009, or possibly early 2010, according to Intel strategist Carlos Weissenberg, speaking during the early part of their roll-out in August 2008[14] (the same view had been stated by market intelligence company DRAMeXchange over a year earlier in April 2007.[15]) The primary driving force behind the increased usage of DDR3 has been new Core i7 processors from Intel and Phenom II processors from AMD, both of which have internal memory controllers: the latter recommends DDR3, the former requires it. IDC stated in January 2009 that DDR3 sales will account for 29 percent of the total DRAM units sold in 2009, rising to 72% by 2011.[16]

Successor

It was revealed at the Intel Developer Forum in San Francisco 2008 that the successor to DDR3 will be known as DDR4. It is currently in the design stage, and is expected to be released in 2012.[17] When released, it is expected to run at 1.2 volts or less,[18][19] versus the 1.5 volts of DDR3 chips and have in excess of 2 billion data transfers per second.